CONTEXT 177 : SEPTEMBER 2023 23 WORKING WITH GLASS An alternative to wire guards that has been used in recent decades is clear polycarbonate sheet. This can provide effective protection even against the smallest projectiles and avoids the shadow lines of wire guards. However, polycarbonate has a relatively short lifetime (manufacturers generally guarantee performance for 10 years) before it starts to deteriorate, gradually losing transparency, and eventually becoming discoloured and opaque. This can greatly detract from the external appearance of the building, giving a blind look to the windows. For this reason, the use of polycarbonate is not recommended by the Church of England (ChurchCare), although it can still be permitted in areas of high vandalism. Protection from environmental damage Environmental damage may be a less obvious threat to stained glass. After all, one function of the windows is to seal the building from the external environment. However, as part of the building envelope, stained glass windows are particularly exposed to the weather, and the potentially damaging effects of sun, wind, rain and pollution. Atmospheric moisture, in particular, whether rain (on the external surface) or condensation (on the internal surface), is a powerful agent of environmental damage. The presence of moisture encourages the corrosion of medieval glass, leading to pitting of the surface and loss of transparency, and the loss of fragile painted detail from both medieval and 19th-century stained glass. Moisture also supports biological growth such as moulds and algae, which hold moisture longer on the surface of the glass, accelerating the rate of deterioration. The installation of internally ventilated environmental protective glazing (EPG) allows air from within a building to pass freely over both sides of the stained glass. The air, being of equal temperature on both sides, keeps the stained glass dry and in as near museum conditions as can be achieved within the architectural setting. In the UK the use of EPG was pioneered some 50 years ago by Barley Studio founder Keith Barley. He was able to assimilate the best ideas and materials found on windows throughout Europe with the help of a Winston Churchill Travelling Fellowship project. The use of EPG is generally agreed to be the best approach to environmental protection for stained glass. Indeed, the international Corpus Vitrearum Guidelines for the Conservation and Restoration of Stained Glass (2nd ed, 2004) state that ‘The installation of a protective glazing system is a crucial part of the preventive conservation of architectural stained glass’. Relieved from their practical function of being a climatic barrier, the protected stained glass windows are displayed as works of art in EPG that significantly retards further deterioration. The installation of EPG brings other benefits, notably that further conservation treatments can be minimised. For example, re-leading (even of weak or buckled panels) can often be avoided and the existing leadwork conserved. This is particularly important for 19th-century stained glass, where the leadwork is likely to be original to the artwork. Non-invasive treatments, such as ‘cold’ painting to replace lost or faded painted detail, can be introduced to enhance the visual appearance. It is also possible to incorporate additional protection along with the environmental protection, for example by using laminated safety glass as the secondary layer for additional mechanical protection (removing the need for wire guards), or using ultra-violet protective glass to protect both the historic glass and the interior of the building. Design of EPG systems Technically, the installation of protective glazing involves the introduction of a second glazing layer outside of the historic glass. Crucially, the installation must allow for ventilation of the interspace between the two glazing layers. Unventilated protection can be damaging, trapping heat, moisture and dirt against the historic glass, especially if the gap between the two layers is minimal. Over 50 years of international research and monitoring has shown that the best environmental conditions for the historic glass are achieved through internal ventilation of the interspace, although there are different ways to achieve this, depending on the particular installation. It is always necessary to consider both the stained glass and its surrounding fabric (stone, brick or timber frame; supporting metalwork; decorative features) in designing an appropriate and effective installation. Internal ventilation ensures that damaging condensation no longer forms on the internal surface of the stained glass, and the reduction Corrosion of 16th-century glass from Lichfield Cathedral, seen in transmitted light (left) and reflected light (right), illustrates the need for environmental protection. (Photo: Barley Studio)
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